1. Field of the Invention
This invention relates generally to startup circuits, which temporarily provide, or cause to be provided, a suitable startup operating voltage for a first subcircuit during the time immediately following application of power to the overall circuit, whereby another subcircuit coupled to and dependent on operation of the first subcircuit may become fully operational and stable prior to removal of the startup voltage; and more specifically to a startup circuit used in a bandgap voltage reference circuit in which the output node of the output stage is coupled to the preceding bandgap circuit and first stage amplifier, thereby improving power supply rejection ratio (PSRR) of the overall circuit.
2. Description of the Related Art
Many electronic circuits are powered by a power supply external to the circuit, thereby causing some operational parameters of the circuit to be dependent on the characteristics of the voltage or current provided by the external power supply. In this document, the voltage and current provided by this external supply are referred to as AVDD, and are typically coupled to circuit terminals also labeled AVDD. An example of one such circuit is a high-gain amplifier, which typically has multiple cascaded gain stages to provide the desired overall gain. It is desirable for the first stage (or stages) of such an amplifier to be powered by a supply voltage having low levels of superimposed noise and ripple, since many typical amplifier stages couple some of this power supply noise to the amplifier output. A known figure of merit for an amplifier is power supply rejection ratio (PSRR), which is a measure of the isolation between the power supply input of the amplifier and the amplifier output. PSRR is also commonly used as a figure of merit for many other circuit types, such as voltage references, voltage regulators, and others.
One known approach to improving PSRR is the use of a subregulated supply. A voltage regulator with its input coupled to the overall supply voltage AVDD is used to produce a subregulated voltage, typically lower than AVDD and quieter than AVDD. A low-noise preamplifier is an example of a circuit which might benefit from a subregulated supply for some or all of the gain stages to increase PSRR.
Another circuit type which benefits from high PSRR is a voltage reference. A high PSRR is desired to keep fluctuations and noise on AVDD from coupling to the output of the voltage reference. A typical voltage reference has a bandgap internal reference voltage generator followed by a buffer amplifier to increase the internal reference voltage to the desired output reference voltage, and to provide a low output impedance. If the bandgap internal reference voltage generator is powered by AVDD, noise on AVDD which is coupled to the internal reference voltage is then amplified and coupled to the voltage reference output. If the bandgap internal reference voltage generator is instead powered by a subregulated supply, PSRR may be dramatically improved. The output of the voltage reference may be used as the subregulated supply voltage for preceding stages such as the bandgap internal reference voltage generator, since the output voltage by design is typically very stable.
However, when the bandgap subcircuit is powered by its own amplified and buffered voltage, startup may not occur reliably or at all. If a stable state exists where the output voltage is lower than the voltage required by the bandgap subcircuit to operate, proper startup may not occur. To preclude such startup problems, a startup subcircuit may provide temporary power to the bandgap subcircuit until internal voltages stabilize at desired levels, after which the startup subcircuit may be electronically removed from the overall circuit.
Known startup circuits apply the AVDD voltage temporarily to subcircuits which, after startup, are powered by a quieter internally generated subregulated voltage. The duration of the application of AVDD may be controlled by a time delay or by the crossing of some threshold by an internal circuit parameter.
An apparatus and method for reliable startup of a circuit using a subregulated supply, having both reliable operation and simple topology, is therefore desirable and is an object of the present invention.